The invention relates generally to a method for detecting tin, and more specifically to a method for detecting tin on surfaces of cast articles, for example, turbine engine components.
Liquid metal cooling is often used to form high-gradient castings of superalloy components in advanced gas turbines, as well as other industrial parts. A molten metallic bath used for the cooling, generally, includes tin. During the cooling process, some of the molten metal can breach the casting container and deposit on the surfaces of a cast article as a contaminant. The cast article is typically subjected to a series of thermal fabrication and heat treatment cycles before becoming a useful article. In certain demanding applications, for example gas turbine engine airfoils, the presence of even trace amounts of low melting residual metal, usually tin, may have severe negative impact upon the surface quality and high temperature properties of the article.
Various cleaning methods are used to remove tin contamination. One example is grit blasting followed by etching in a chemical bath. These two processes are effective in removing most of the tin. However, thin residual layers may remain. These residual layers are poorly discernable to the naked eye. The use of advanced coating removal (ACR) baths has also been proven to be highly effective to remove tin. However, confirmation of tin removal is critical to subsequent processing.
Different processes, chemical or non-chemical, have been proposed to detect residual tin. For example, on heating at a high temperature, residual tin, if present, transforms into a tan-colored matter that can be easily seen with the naked eye. In this process, articles are subjected to a costly and time-consuming heating process needlessly if they had no residual tin. Other detection processes, such as use of an X-ray fluorescence analyzer, a colorimetric reagent provide ability to visualize residual tin immediately after cleaning process. However, most of these methods are either very expensive, complicated or include reagents that may damage the article surface.
Accordingly, there is a need for methods that enable quick and inexpensive detection of tin. It would also be desirable if the method does not result in the formation of an unacceptable amount of hazardous matter. For example, the method should effectively detect tin while substantially preserving the article.